For many commercial end-uses, the melt-flow characteristics of C.sub.3 + polyolefins, especially polypropylene, are not suitable because of the relatively high molecular weight (MW) of such polymer as it is originally produced in the synthesis process. Important end-uses where it has become well accepted that the melt flow characteristics of such polymers must be substantially improved are in fibers and/or films as well as in various extruded and injection- and blow-molded product grades.
In view of this need, it has been shown in the past that higher melt-flow characteristics can be achieved by controlled chain scission, which in effect reduces the molecular weight of the longer and thus high molecular weight chains. The average MW is reduced lowering the melt viscosity. Furthermore, the molecular weight distribution (MWD) is significantly altered, primarily because of the reduction of the high MW fraction. Improvement of melt properties associated with reduction of melt elasticity results in reduced die swell in extrusion and reduced spin resonance in fiber spinning. This chain cleavage is normally accomplished by oxygen and/or free radical sources such as peroxides.
The basic concept of accomplishing such degradation by utilizing peroxides is claimed in U.S. Pat. No. 3,144,436, where a free radical initiator is introduced into a polymer melt in the absence of oxygen in a screw extruder. U.S. Pat. No. 3,940,379 discloses a method for the degradation of propylene polymers to increase their melt flow rate which comprises contacting a propylene polymer with oxygen and an organic or inorganic peroxide, melting and working the resulting mixture in a high shear zone, and recovering an essentially odor-free propylene polymer. U.S. Pat. No. 4,061,694 discloses the manufacture of propylene molding compositions of improved impact strength by subjecting block copolymers of ethylene and propylene to controlled oxidative degradation under conditions essentially similar to those of the preceding patent. Other patents dealing with degradation of polypropylene include U.S. Pat. Nos. 4,375,531; 3,862,265; 3,887,534; 3,898,209; 4,001,172; 4,087,486; 4,359,495; 4,378,451 and 4,387,185.
A new term has been coined for such degraded or cracked polypropylene, that term being "controlled rheology" (CR) polypropylene. Although controlled rheology polypropylene has been commercially available for several years, its similarities and differences from "normal" or reactor polypropylene are just starting to be understood. CR polymers have a variety of advantages and disadvantages. The growing diversity of the polyolefin market is putting an increasing demand on polyolefin manufacturers for product grades to fit a large variety of processing behaviors as well as bulk mechanical properties. Increased control over MW and MWD in the manufacturing process is a powerful step in this direction.
Typically, the polypropylene producers have focused on the single property, "melt flow", when manufacturing CR polypropylene for specific products. However, another property molecular weight distribution (MWD) is also critically important. As discussed by Brown et al in "Molecular Weight Distribution and its Effect on Fiber Spinning", Fiber World, Vol. 1, No. 2, pages 32-43 (March 1984), the three commonly used molecular weight averages are M.sub.n, M.sub.w, and M.sub.z. These are obtained by three different averaging methods, referred to as "number", "weight", and "z" and are based on ratios of successively higher moments of the MWD. The MWD itself can be defined by various ratios of these averages, as follows:
Q=M.sub.w /M.sub.n PA1 R=M.sub.z /M.sub.w PA1 S=M.sub.z /M.sub.n
In some cases, these are inadequate to express a detailed description of the MWD as they are based on averaging processes. In this case a detailed "spectral analysis" of the MWD is preferable, where separate segments of the MWD are specifically examined.
Polypropylene homopolymers of different MF have roughly the same shape of MWD when prepared by the same reactor process. With a CR resin, this MWD changes. The three molecular weight averages are all reduced in the CR process. The fastest changing average is M.sub.z, while the slowest is M.sub.n. This is not surprising since the high molecular weight end of the MWD is the most changed by the CR process. Also not surprising is that S decreases faster than Q, which decreases faster than R. After the CR process takes place, a MWD becomes skewed due to the preferential loss of the high molecular weight components. For a completely random scission process, Q approaches a limiting value of 2.0, while R approaches 1.5.
Rheological (melt flow) behavior is very sensitive to the MWD, particularly to the high molecular weight portion of the MWD. Reduction of the high molecular weight portion of the MWD with corresponding increase of the medium or low molecular weight portions of the MWD is referred to as "narrowing" of the MWD. The difference between "narrow" and "broad" MWD can have profound effects on melt processibility. For example, for two polypropylenes with the same melt flow index, but having different breadth of MWD, the polypropylene with the narrow MWD will generally show a reduced shear sensitivity over a wider shear range than that with the broad MWD. In the past it has not been possible to achieve independent variability of melt flow and molecular weight distribution without blending together various CR polymers or using different polymers from different polymerization conditions. I have discovered a new method that permits the preparation of increased melt flow products along with control over the desired molecular weight distribution, without blending, using a single degradation or cracking process.